1. Field of the Invention
The present invention relates to an image sensing apparatus, control method thereof, and storage medium, particularly an electronic zoom technique of reducing the degradation of resolution.
2. Description of the Related Art
Recent advanced image sensing elements have significantly developed image input devices including an electronic still camera and video camera. In particular, electronic still cameras and video cameras with a movie function have been proposed. A camera of this type has an electronic zoom function of extracting only signals of a desired range from image signals read out from an image sensing element, and enlarging the image of the extracted signal to zoom electronically.
The electronic zoom function is designed to make the readout pixel count almost equal to the output pixel count for display and recording at the widest angle of view. As the camera zooms in, the readout pixel count runs short compared to the output pixel count, degrading the resolution. As solutions to this, the following methods have been proposed.
The first method provides an electronic zoom which hardly degrades the resolution. According to this method, signals are always read out from all the pixels (e.g., 1,280×960) of a region corresponding to the widest angle of view. For a telephoto angle of view, some of the readout signals are extracted and directly read out (e.g., 640×480 for a 2× zoom). For a wide angle of view, signals are output by reducing the pixel count by conversion (in this case, reduction conversion from 1,280×960 to 640×480).
However, the first method needs to always read out signals from all the pixels (1,280×960), decreasing the display frame rate (to 1/4 of the frame rate to read out signals from 640×480 pixels).
The second method is an electronic zoom method which makes the readout pixel count almost equal to the output pixel count for display and recording, reducing the degradation of resolution, like Japanese Patent Laid-Open No. 2000-295530. According to this method, for example, when a 2× zoom function is activated, signals are read out from all the 640×480 pixels of an extracted region for a telephoto angle of view. For a wide angle of view, signals are read out from 640×480 pixels out of the entire 1,280×960 region while thinning them at a thinning rate of 1/2 vertically and horizontally. The entire 1,280×960 region is used to, for example, shoot a still image.
However, the second method can take only discrete magnifications capable of thinning readout. When the electronic zoom is used, images can only be shot at discrete magnifications.
The third method is an electronic zoom method of gradually changing the thinning rate of image signals output from an image sensing element, instead of simple thinning, in order to attain an intermediate magnification, like Japanese Patent Laid-Open No. 2005-191867. This method reads out image signals from n pixels out of m pixels (m≧n; m and n are natural numbers) at a thinning rate n/m. Changing n/m implements an intermediate magnification.
The third method can provide an intermediate magnification by gradually changing the thinning rate of image signals. However, this method needs to always read out signals from all the pixels and suffers a low display frame rate, similar to the first method.
In view of this, Japanese Patent Laid-Open No. 2002-330329 proposes the fourth method. According to this method, images of the predetermined size necessary for display are read out from a region D and larger region E at a thinning rate d and higher thinning rate e. For an intermediate region between the regions D and E, an image is read out from the region E at the thinning rate d, and converted into the predetermined image size necessary for display using the image size of the intermediate region out of the readout image. As a result, an electronic zoom at an intermediate magnification is achieved. Note that thinning to 1/e is expressed as a thinning rate e in Japanese Patent Laid-Open No. 2002-330329, and will be defined as a thinning rate of 1/e in this specification.
The fourth method attains an electronic zoom by reading out an image from the region D at the thinning rate d, and even for an intermediate region between the regions D and E, reading out an image from the region E at the thinning rate d. In this case, the readout image size (pixel count) of the region E becomes the square of e/d of the readout image size (pixel count) of the region D. The readout time of the region E also becomes the square of e/d of the readout time of the region D, and limits the frame rate in movie shooting.
For example, when the predetermined image size necessary for display is 10×10 pixels, an image is read out from a 10×10 pixel region at a thinning rate of 1, and read out from a 20×20 pixel region at a thinning rate of 2. In an intermediate region between the 10×10 pixel region and the 20×20 pixel region, an image is read out from the 20×20 pixel region at a thinning rate of 1. The readout pixel count becomes the square of 2/1, that is, quadruples.
The readout time is unbalanced between readout of the region E and that of the region D. When the electronic zoom is executed, the readout time of a shot image differs between regions.
For example, the readout timing of the first pixel (1st pixel) in the 10×10 pixel region and that of the first pixel (1st pixel) in the 20×20 pixel region differ from each other by a time corresponding to readout of 300 pixels. The readout timing of the final pixel (100th pixel) in the 10×10 pixel region and that of the final pixel (400th pixel) in the 20×20 pixel region differ from each other by a time corresponding to readout of 300 pixels.
Japanese Patent Laid-Open No. 2002-330329 describes a technique of reading out an image from a region corresponding to a desired intermediate magnification by changing the readout method of pixels from an image sensing element while implementing an image size necessary for display at the intermediate magnification by the same method as the third one.
However, control becomes complicated if the readout method of the image sensing element is changed at every intermediate magnification of the electronic zoom.